Aluminum hydride, also referred to as "alane," is usually prepared as a solution by the reaction of lithium aluminum hydride with aluminum trichloride. A. E. Finholt et al. (1947) J. Chem. Soc. 69:1199. The alane-containing solution, however, is not stable, as an alane-ether complex precipitates from solution shortly after preparation. In addition, attempts to isolate the nonsolvated form of alane from the ether solution result in the decomposition of the complex to aluminum and hydrogen. M. J. Rice Jr. et al. (1956) Contract ONR-494(04) ASTIA No. 106967, U.S. Office of Naval Research.
In a method for preparing non-solvated alane, alane-etherate may be desolvated in the presence of a small amount of lithium aluminum hydride. See, for example, A. N. Tskhai et al. (1992) Rus. J. Inorg. Chem. 37:877, and U.S. Pat. No. 3,801,657 to Scruggs. Non-solvated alane exhibits six crystalline phases, with each having different physical properties. The phase designated as .alpha.'-alane is essentially non-solvated and appears under a polarizing microscope as small multiple needles growing from single points to form fuzzy balls. The .gamma. phase appears as bundles of fused needles. The .gamma. phase is produced in conjunction with the .beta. phase, and both .gamma.- and .beta.-alane are metastable nonsolvated phases that convert to the more stable .alpha.-alane upon heating. The .alpha.-alane is the most stable, and is characterized by hexagonal or cubic shaped crystals that are typically 50-100 .mu.m in size. The other two forms, designated .delta.- and .epsilon.-alane, are apparently formed when a trace of water is present during crystallization, and the .zeta.-alane is prepared by crystallizing from di-n-propyl ether. The .alpha.', .delta., .epsilon. and .zeta. polymorphs do not convert to the .alpha.-alane and are less thermally stable than the .alpha.-form. For a discussion of the various polymorphs, reference may be had to F. M. Brower at al. (1976) J Am. Chem. Soc. 98:2450.
Alane consists of about 10% hydrogen by weight, thereby providing a higher density of hydrogen than liquid hydrogen. Because of the high hydrogen density and the highly exothermic combustion of aluminum and hydrogen, alane can be used as a fuel for solid propellants or as an explosive.
Solvated alane can be synthesized by the reaction of LiAlH.sub.4 with aluminum chloride, resulting in the alane.etherate complex (equation 1). ##STR1##
In an alternative synthesis, LiAlH.sub.4 is reacted with sulfuric acid to give the alane.etherate complex (equation 2). ##STR2##
The AlH.sub.3 -ether complex is then treated with a mixture of LiAlH.sub.4 and LiBH.sub.4, and heated (equation 3). ##STR3##
The combination of LiBH.sub.4 /LiAlH.sub.4 enables use of a lower processing temperature, and .alpha.-alane is the final product after heating at 65.degree. C. under vacuum. In an alternative synthesis, Bulychev reports that .alpha.-alane can be prepared at pressures greater than 2.6 GPa and at temperatures in the range of 220-250.degree. C. B. M. Bulychev et al. (1998) Russ. J. Inorg. Chem. 43:829. Under those conditions, apparently only the .alpha.-alane form is observed.
In addition, alane can be directly synthesized by metathesis of aluminum alkyls followed by removal of the alkylaluminum byproduct in vacuum (equation 4). ##STR4##
Still another method of preparing nonsolvated alane is by bombarding an ultrapure aluminum target with hydrogen ions. However, alane thus produced has poor crystallinity.
One of the obstacles to large scale production of .alpha.-alane is the handling of the diethyl ether solution of the alane.cndot.ether complex. At concentrations of about 0.5 M or higher and temperatures above 0.degree. C. the alane.cndot.ether phase prematurely precipitates out of solution. In addition, .alpha.-alane can be contaminated with other phases of alane, and is not stable over time as the complex decomposes to hydrogen and aluminum.
Thus, although alane is potentially promising as a high energy density fuel, because of its high hydrogen density and the highly exothermic combustion of aluminum and hydrogen, the lack of a suitable method for synthesizing alane in a stabilized form has severely limited its applicability.